Various proposals have been made to enable information to be obtained concerning the position of a helmet in space to be used for automatic sighting of a missile. Thus, it is known to provide on the helmet radiation sources which are arranged to emit radiation which can be intercepted by sensing means coupled to suitably programmed computing means so as to determine the line of sight of the helmet. U.S. Pat. No. 4,111,555 (Elliott Brothers (London) Ltd.), for example, describes such a system wherein there are provided on the helmet two sets of light emitting diodes (L.E.Ds) arranged in a triangular formation. The sensing means comprises, generally, two independent linear arrays of light-sensitive charge-coupled devices, each of which is sensitive to the radiation emitted by at least one set of L.E.Ds.
The helmet line of sight is determined when the pilot sights a target through a reticle fixed on the helmet's visor. Computing means coupled to the sensors is programmed to determine the helmet line of sight from a knowledge of the positions on the two sensors of the three L.E.Ds of at least one set of L.E.Ds. In this context, the helmet line of sight corresponds to the direction of a line joining a fixed point of origin on the helmet with the reticle.
There are several disadvantages with such a system. Owing to the fact that each sensor is linear, means must be provided for determining which particular L.E.D. is being imaged and, to avoid ambiguity, either L.E.Ds of different frequency must be employed or the angular positions of the L.E.Ds must be sensed one at a time. The former solution demands that frequency discrimination means be associated with the sensors whilst the latter assumes that the time interval between the angular positions of successive L.E.Ds being sensed by the two sensors is sufficiently small that the helmet remains substantially stationary during this time interval.
A further disadvantage with such a system is the requirement to provide two independent sensors. Additionally, such a system is intended to measure the angular displacement only of the helmet whereas it would be preferable to determine all six spatial coordinates of the line of sight of an object, corresponding to the three directional coordinates, as well as the three cartesian coordinates of the reference point of the line of sight.